Manufacturing micro-structured elements

ABSTRACT

An assembly including a joining element and an object, the object having a first outer layer, a second outer layer, and a middle layer between the first and second outer layers. The object has an opening that extends through the first outer layer and the middle layer, and the joining element is anchored in the opening. The joining element includes a proximal end, a distal end, a contact face in the area of the proximal end, a wall anchoring portion, and a tubular portion. The contact face is engaged by a vibrating tool that applies mechanical vibration and axial force to the joining element. The joining element includes a thermoplastic material at least at the circumferential surface of the wall anchoring portion and at the distal end to anchor the joining element to the first and second outer layers, respectively.

This application is a divisional of U.S. application Ser. No. 14/271,783filed May 7, 2014, which is a continuation of U.S. application Ser. No.14/023,573 filed Sep. 11, 2013, currently pending, which is a divisionalof U.S. application Ser. No. 12/827,699 filed on Jun. 30, 2010, now U.S.Pat. No. 8,550,759, which is a division of U.S. application Ser. No.11/962,292, filed on Dec. 21, 2007, now U.S. Pat. No. 7,765,672, whichis a non-provisional of U.S. Provisional Application No. 60/882248 filedDec. 28, 2006

BACKGROUND OF THE INVENTION

The invention concerns a method for anchoring a joining element in anobject and a joining element to be used in the method. The joiningelement comprises a thermoplastic material (or a material having athermoplastic component) and anchoring is effected with the aid of thethermoplastic material and mechanical vibration (e.g. ultrasonicvibration) through which the thermoplastic material is at least locallyliquefied. The joining element serves for connecting a further object tothe named object in which it is anchored or the joining element is partof such further object.

It is known from the publications WO 98/42988 and WO 00/79137, to anchorjoining elements in objects with the aid of a thermoplastic materialwhich is liquefied by mechanical vibration, and which, in the liquefiedstate, is pressed into suitable surface structures in the object, whereon re-solidification it forms a positive fit connection with the namedsurface structures and therewith anchors the joining element in theobject. The material of the object in which the joining element is to beanchored possesses e.g. an open porosity or the surface of the object ina region where anchorage is to be affected comprises a plurality ofcorresponding cavities or protrusions. The thermoplastic material of thejoining element usually has an elasticity module which is greater than0.5 GPa and therefore this material can be liquefied by mechanicalvibration only if contact with the object is limited to a plurality ofsmall locations (energy directors). This is achieved e.g. by providingenergy directors in the form of elements protruding from a main surfaceof the joining element which protruding elements taper to an edge ortip.

In many cases in which a joining element is anchored in an object withthe aid of a thermoplastic material and mechanical vibration, an openingis provided in the object and it is desired that the joining element isanchored either in the walls of the opening or in the walls and in thebottom of the opening. Anchorage in the walls of the opening is achievedusually by dimensioning the joining element and the opening such thatthe circumference of a cross section of the joining element includingenergy directors is somewhat larger than the corresponding circumferenceof the cross section of the opening, such that on introducing thejoining element into the opening, the circumference of the joiningelement is at least locally (energy directors) pressed against the wallof the opening.

In particular, if anchorage in the walls of the opening is desired inthe immediate vicinity of the mouth of the opening, it often happensthat the liquefied material interpenetrates the surface of the object inthe vicinity of the opening and small amounts of the material protrudeonto this surface. This effect blurs the contours of the opening and/orof the joining element in the mouth region such that these contours donot appear as clear cut as this may be desired for various reasons. Inparticular, if a proximal face of the joining element, which is anchoredin an opening provided in the object, is to be flush with the objectsurface it is desirable mainly for esthetic reasons that thecircumference of the joining element and of the opening are clearlydefined, i.e. not blurred at all.

Anchorage in the immediate vicinity of the mouth of the opening is inmany cases desired and it is necessary, if the object is very thin or ifthe opening is very shallow. This is the case if the object is asandwich board with thin outer layers and an inner layer, whereinanchorage is possible only in the outer layers. An example of such aboard is a so called hollow core board comprising outer layers of woodor a wood-like material and an inner layer being constituted by ahoneycomb structure of cardboard, in which the named anchorage is notpossible.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to create a method for anchoring ajoining element in an object and a joining element to be used in themethod, wherein for anchoring the joining element, an opening isprovided in the object, wherein the joining element comprises athermoplastic material (or a material having a thermoplastic component)and anchorage is achieved by liquefaction of the thermoplastic materialthrough mechanical vibration being applied to the joining element whenit is forced into the opening. The anchoring method and the joiningelement according to the invention are to enable anchorage of thejoining element in the wall of the opening, in particular in theimmediate vicinity of a mouth of the opening wherein the contours of themouth and the joining element at the level of the mouth are to remain asclear cut when the joining element is anchored as before anchoring.

This object is achieved by the anchoring method and the joining elementas defined in the corresponding independent claims. Preferredembodiments are defined in the dependent claims.

For anchoring the joining element in a substantially cylindrical orslightly conical wall (having a round or non-round cross section) of anopening in the object, which wall extends away from the mouth of theopening, the joining element comprises a substantially cylindrical orslightly conical wall anchorage portion which is adapted regarding formand size to the named wall and whose circumferential surface comprisesthe thermoplastic material and is equipped with energy directors, i.e.with ribs and/or bumps which taper towards the outside in more or lesssharp edges or tips. For anchorage, this wall anchorage region, whosecross section is slightly larger than the cross section of the mouth andopening wall is forced through the mouth by applying mechanicalvibration (e.g. ultrasonic vibration) to a proximal end of the joiningelement and at the same time forcing the wall anchoring portion of thejoining element through the mouth of the opening. Thereby thethermoplastic material of the circumferential surface of the wallanchoring portion is at least partly liquefied and is pressed intopores, unevenesses or specifically provided cavities of the walls and onre-solidification anchors the joining element therein.

For preventing the liquefied material from protruding through the mouthof the opening on to the surface of the object in which the opening isprovided, a sealing means is forced through the mouth immediatelyfollowing the wall anchoring portion of the joining element, the sealingmeans being again substantially cylindrical or slightly conical andhaving a cross section which corresponds in form to the cross section ofthe mouth of the opening but is slightly larger in cross section thanthe latter such that it needs to be forced into the mouth and effectssealing by a press-fit. The circumferential surface of the sealing meansis smooth for effective sealing and, if the sealing means consists ofthe thermoplastic material, for preventing liquefaction thereof (noenergy directors).

As mentioned above, the cross section of the sealing means has the sameform as the cross section of the mouth and is on all sides slightlylarger than the latter by about 0.1 to 1 mm. Furthermore, the crosssection of the sealing means envelopes the cross section of the wallanchoring portion of the joining element, i.e. the edges and tips of theenergy directors provided on the circumferential surface of the wallanchoring portion have the same distances from a joining element axis asthe corresponding part of the circumferential surface of the sealingmeans.

In a preferred embodiment of the joining element according to theinvention, the sealing means is part of the joining element, i.e. it isarranged adjoining the wall anchoring portion on the proximal sidethereof, either directly or with a narrow groove therebetween, thegroove region having a smaller cross section than the sealing means.However, the sealing means may also be constituted by the distal end ofa tool which is applied to the proximal end of the joining element forcoupling vibration and force into the joining element.

A joining element to be used in the method according to the inventioncomprises a thermoplastic material, preferably with a modulus ofelasticity of more than 0.5 GPa. The joining element further comprises asubstantially cylindrical wall anchoring portion between a distal endand a proximal end distanced from each other along a joining elementaxis. The joining element is dimensioned such that, on the anchoredjoining element, the wall anchoring portion is situated inside of themoth of the opening. The region of the proximal end comprises a contactface suitable for being contacted with a vibrating tool (e.g. asonotrode of an ultrasonic device), which contact face advantageouslyextends perpendicular to the joining element axis. The thermoplasticmaterial forms at least part of the circumferential surface of thejoining element in the wall anchoring portion. The distance between thedistal end of the joining element and the proximal end of its wallanchoring portion substantially defines a depth to which the joiningelement is to be introduced and anchored in the opening.

A preferred embodiment of the joining element to be used in the methodaccording to the invention also comprises the sealing means which, as asealing portion, is arranged coaxially adjoining the wall anchoringportion at its proximal side, the cross section of the sealing portionenveloping the cross section of the wall anchoring portion, wherein thecircumferential surface of the sealing portion is smooth and thecircumferential surface of the wall anchoring portion comprises aplurality of recesses forming between them a plurality of energyconcentrating elements, i.e. a plurality of rib- or hump-shaped elementstapering off to edges or tips. The smooth circumferential surface of thesealing portion and the recess containing surface of the wall anchoringportion meet along a border line or narrow border groove running aroundthe circumference of the joining element, which border line or groove isadapted to the shape of the surface of the object in which the openingis provided. For instance, if the named object surface is even and theopening extends substantially perpendicular to this surface, the borderline extends in a plane which is perpendicular to the joining elementaxis.

The joining element is forced into the mouth of the opening provided inthe object until the sealing portion has passed the mouth and issituated at a distance from the mouth of at least 0.5 mm preferably ofat least 1 mm.

The sealing portion of the joining element or even the whole joiningelement may consists of the thermoplastic material. The smoothcircumferential surface of the sealing portion which is in contact withthe wall of the opening in the anchoring process does not enableliquefaction because there are no places where the vibrating energy isconcentrated. However, it is possible also to use non-thermoplasticmaterials or thermoplastic materials melting at much higher temperaturesor metals for the sealing portion and also for a core region of thejoining element.

The wall anchoring portion of the joining element may continue to thedistal end thereof. Alternatively, the joining element may furthercomprise a distal end region adjoining the wall anchoring portion on itsdistal side and having another shape and/or surface than the latter,wherein this distal end region, for being able to pass through the mouthof the opening, has a smaller cross section than the wall anchoringportion. The sealing portion of the joining element may form theproximal end of the joining element. Alternatively, the joining elementmay further comprise a proximal end region adjoining the sealing portionand having another shape and/or surface than the latter, wherein thisproximal end region may have a larger or smaller cross section than thesealing portion.

If the joining element is to be anchored in the opening with itsproximal face flush with the object surface in which the mouth of theopening is situated, then the sealing portion has a small axial lengthand carries the proximal contact face. In such cases, the axial extentof the sealing portion, i.e. the distance between the above definedborder line and the proximal face is preferably at least 0.5 mm,preferably at least 1 mm.

It is possible also that in the method according to the invention thesealing function is taken over by a distal end region of a tool used forpressing the joining element into the opening provided in the object andfor coupling the mechanical vibration into the joining element. In suchcases, the wall anchoring portion of the joining element forms itsproximal end and is forced with the tool to just below the mouth of theopening in the object.

BRIEF DESCRIPTION OF THE DRAWINGS

The method for anchoring the joining element in the object and aplurality of exemplary embodiments of the joining element according tothe invention are described in detail in connection with the followingfigures.

FIG. 1A is a perspective view of the joining element;

FIG. 1B is a cross sectional plan view through a wall anchoring region;

FIG. 1C is an elevation view illustrating the joining element beinganchored in an opening provided in the object;

FIG. 1D is an elevation view illustrating detail of a proximal end ofthe wall anchoring region;

FIG. 1E is an elevation view illustrating detail of the proximal end ofthe wall anchoring region;

FIG. 2A is an elevation view of the joining element;

FIG. 2B is cross sectional plan view of the wall anchoring portion;

FIG. 2C is a elevation view of the joining element being anchored in anopening provided in the object;

FIG. 3A is a plan view illustrating a hollow core board before theanchoring process;

FIG. 3B is a plan view illustrating the hollow core board after theanchoring process;

FIG. 4 is a perspective view illustrating a further exemplary embodimentof the joining element according to the invention;

FIG. 5 is an elevation view illustrating a joining element and tool forcarrying out the method according to the invention;

FIG. 6 is an elevation view illustrating the joining element and toolfor carrying out the method according to the invention; and

FIG. 7 is an elevation view illustrating the joining element and toolfor carrying out the method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A to 1C illustrate a first exemplary embodiment of a joiningelement according to the invention. Furthermore, FIGS. 1D and 1E showtwo different embodiments of a proximal end of a wall anchoring portionof the joining element.

The joining element consists fully of the thermoplastic material andcomprises a proximal end 1 and a distal end 2, wherein the proximal end1 and the distal end 2 are distanced from each other along a joiningelement axis A. Between the proximal end 1 and distal end 2, the joiningelement comprises a mouth region 3, which is the region where a sealingportion 3.1 and a wall anchoring portion 3.2 meet. The mouth region 3 issubstantially cylindrical or slightly conical and has a cross section,as shown in FIG. 1B as substantially round. The proximal end 1 comprisesa contact face 1.1 suitable for applying a tool for coupling mechanicalvibration and a force in the direction of the joining element axis Ainto the joining element. The proximal end 1 may also comprise means forattaching the joining element to the tool or for guiding the tool, e.g.an axial recess 1.2. The distal end 2 is adapted to the opening in whichit is to be anchored, the opening 5 provided in the object 10 being e.g.a blind bore.

The circumferential surface of the sealing portion 3.1 is smooth. Thecircumferential surface of the wall anchoring portion 3.2 comprises aplurality of recesses 4.1 (e.g. in a regular pattern) and betweenrecesses 4.1, a plurality of energy concentrating elements 4.2 in theform of axial ribs tapering off to more or less sharp edges, which havesimilar distances from the joining element axis A as the smoothcircumferential surface of the sealing portion 3.1. This means that thecross section of the sealing portion 3.1 envelopes the cross section ofthe wall anchoring portion 3.2 (including energy concentrating elements)as illustrated in FIG. 1B, which is a cross section through the wallanchoring portion 3.2 viewed towards the sealing portion 3.2. Thesealing portion 3.1 and the wall anchoring portion 3.2 meet in a mouthregion 3 of the joining element along a border line 6, which lies in aplane perpendicular to the joining element axis A.

For anchoring the joining element according to FIGS. 1A and 1B, theblind bore (opening 5) is provided in the board 10, wherein a bore axisA′ is oriented substantially perpendicular to the surface of the board10 and wherein, of the bore 5, at least the mouth 5.1 and a piece of thewall extending away from the mouth 5.1 have a cross section whichregarding form is adapted to the cross section of the sealing portion3.1 of the joining element, but is slightly smaller than the latter(difference between diameters preferably between 0.2 and 2 mm). If thejoining element is to be anchored not only in the bore walls but also inthe bottom of the bore 5, the depth of the bore 5 is smaller than thedistance between the distal end of the joining element and the borderline 6.

For being anchored in the opening 5, the joining element is positionedin or on the mouth 5.1 of the opening 5 and using a suitable tool (e.g.the sonotrode of an ultrasonic device) which is positioned on andpressed against the contact face 1.1 of the joining element, the joiningelement is forced into the opening 5 while being vibrated. The ribs(energy concentrating elements 4.2) are pressed against the wall of theopening 5 and the thermoplastic material is liquefied there and ispressed into the porous structure of these walls. When the wallanchoring portion 3.2 and the border line 6 have passed the mouth 5.1the distal end of the sealing portion 3.1, which is not equipped withenergy concentrating elements (smooth circumferential surface) is forcedinto the mouth 5.1 wherein, because of the lack of energy directors,hardly any material is liquefied such that this sealing portion 3.1 isable to seal the mouth 5.1 in a clean manner as illustrated in FIG. 1C.The border line 6 which is shown in FIG. 1C as a dash-dotted line is inreality hardly distinguishable after the anchoring process.

As shown in FIGS. 1D and 1E, the border line 6 may be a sharp line asillustrated in FIG. 1D, or it may be a narrow grove (border groove 6.1)extending between the sealing portion 3.1 and the wall anchoring portion3.2 and serving for accommodating a surplus of liquefied material.

Possible alternative features of the joining element and the anchoringmethod as illustrated in FIGS. 1A to 1E are listed hereinbelow.

The cross section of the mouth 5.1 and the sealing portion 3.1 are notround (see also FIGS. 2A to 2C).

The sealing portion 3.1 has a considerably smaller axial length and thejoining element is to be forced into the opening 5 until the contactface 1.1 or a proximal face is flush with the surface of the board 10.

The sealing portion 3.1 does not extend to the proximal end of thejoining element and the region of the proximal end 1 is designed as afitting to be attached to the board 10 or a means for fixing such afitting.

The energy concentrating elements 4.2 of the wall anchoring portion 3.2are not axially extending ribs, but differently oriented ribs orhump-shaped elements (see also FIGS. 2A and 2B).

The joining element does not consist fully of the thermoplasticmaterial, but comprises a core made of e.g. a metal.

The joining element is hollow (see also FIGS. 3A, 3B and 4).

The object in which the joining element is anchored is not a woodenboard 10 or a chip board but consists of another porous material (e.g.concrete, sand stone, metal foam, sintered ceramic or sintered metal) orit consists of a non-porous material and comprises, at least on thewalls of the opening 5, surface structures suitable for beinginterpenetrated by the liquefied thermoplastic material of the joiningelement.

The opening 5 reaches right through the board.

The axis A′ of the opening 5 is oriented at an angle other than a rightangle relative to the surface of the board 10 and the border line 6 ofthe joining element lies in a plane at a same angle relative to thejoining element axis A.

Finally, the surface of the object in which the opening is provided isnot even but, curved and the border line 6 is similarly curved.

FIGS. 2A to 2C illustrate in the same manner as FIGS. 1A to 1C, afurther exemplary embodiment of the joining element according to theinvention and the anchorage thereof in an opening 5 provided in anobject, e.g. in a wooden board 10. As mentioned already further above,the joining element according to FIGS. 2A to 2C comprises a cylindricalmouth region 3 with a substantially square cross section (without sharpcorners), the energy concentrating elements 4.2 of the circumferentialsurface of the wall anchoring portion 3.2 of the mouth region 3 are onthe one hand humps tapering off to a tip and on the other hand, sharpedges of the square cross section forming axially extending ribs. Theregion of the proximal end 1 of the joining element consistssubstantially of a contact face 1.1 only, which contact face 1.1terminates the sealing portion 3.1, which has an axial length of atleast 0.5, preferably at least 1 mm. The joining element is forced intothe opening 5 to a depth such that the contact face 1.1 is flush withthe surface of the object in which the opening 5 is provided.

The region of the distal end 2 of the joining element according to FIGS.2A to 2C comprises a step-like reduction 2.1 in cross section which isadapted to a similar step 5.2 in the opening 5. This results inanchorage not only in the wall of the opening 5 in the vicinity of themouth 5.1 but also on the step 5.2 and possibly also on the bottom ofthe opening 5.

FIGS. 3A and 3B illustrate a further embodiment of the joining elementaccording to the invention and anchorage thereof in a hollow core board20. Both FIGS. 3A and 3B are axial sections wherein FIG. 3A shows thejoining element before being forced into the opening 5 while beingvibrated and FIG. 3B thereafter.

The hollow core board 20 comprises a first outer layer 20.1 of e.g.wood, a second outer layer 20.2 of e.g. wood and a middle layer 20.3consisting e.g. of a cardboard honeycomb. The first and second outerlayers 20.1, 20.2 have a thickness of 3 mm, the middle layer 20.3 of 32mm. The bore 5 penetrates the first outer layer 20.1 and the middlelayer 20.3. The joining element is to be anchored in the walls of theopening in the first outer layer 20.1 and preferably on the innersurface of the second outer layer 20.2. The joining element comprises acentral tube 2.1 reaching to the distal end 2 of the joining element.Within the central tube 2.1 an inner metal core piece (not shown) may bearranged having an inner thread for fixing a further object on thehollow core board 20.

The joining element again comprises a cylindrical sealing portion 3.1and a corresponding wall anchoring portion 3.2 which together form acollar on the central tube 2.1. The sealing portion 3.1 has an axialextension of 1 mm and the wall anchoring portion 3.2 has an axialextension of 2.5 mm and its circumferential surface is equipped withrecesses and axially extending ribs between the recesses, wherein thedepth of the recesses is at least 0.2, preferably 0.3 mm.

The central tube 2.1 has a smaller diameter than the mouth region 3 andtapers off to an edge 21 constituting the distal end 2 of the joiningelement.

The joining element according to FIGS. 3A and 3B is positioned in themouth of the bore 5, wherein the dimensions of the joining element andthe bore 5 are matched such that the wall anchoring portion 3.2 sits onthe mouth 5.1 when the joining element is positioned but can be forcedinto the mouth 5.1 on application of vibration and force parallel to theaxis A, and such that the edge 21 touches the inner surface of thesecond outer layer 20.2, when the joining element is positioned. Thejoining element is then forced into the opening 5, wherein thethermoplastic material is liquefied in the area of the edge 21 anchoringthe joining element in the second outer layer 20.2 of the hollow coreboard 20 and in the area of the wall anchoring portion 3.2 anchoring thejoining element in the walls of the opening through the first outerlayer 20.1 of the hollow core board 20.

For guiding the joining element and for enhancing anchorage in thesecond outer layer 20.2, a blind or through bore may be providedtherein, the axis of such bore being aligned with the axis of the borein the first outer layer 20.1.

The joining element is forced into the opening 5 until the contact face1.1 thereof is flush with the outer surface of the first outer layer20.1 of the hollow core board 20, i.e. until the border line 6 issituated at 1 mm inside the mouth. Experiments show that on removal ofthe vibrating tool from the contact face 1.1, the contours of thejoining element and of the mouth as seen from the outer surface of thefirst outer layer 20.1 of the hollow core board 20 are not blurred atall, but are as clear cut as before the anchoring process.

The hollow core board 20 according to FIGS. 3A and 3B comprises a middlelayer 20.3 comprising e.g. a honeycomb structure of card board. Otherknown middle layers of similar hollow core boards are foamed plasticsand other similar materials. However, it is advantageous also to use thejoining elements as sole spacers between the first and second outerlayers 20.1 and 20.2 such that the middle layer 20.3 is constituted bynothing more than a plurality of such joining elements. If the board isrelatively small all the joining elements may be positioned in locationswhere further objects (e.g. fittings, hinges, other hollow core boards)are to be fixed on the board. If the board is larger, it is advantageousto provide more joining elements, wherein some of the joining elementsserve solely as spacers.

For manufacturing a hollow core board 20 whose middle layer 20.3 isconstituted solely by joining elements as described above, prior tointroduction and anchoring of the joining elements correspondingopenings through the first outer layer 20.1 are to be provided and thetwo outer layers 20.1 and 20.2 are to be positioned at the desireddistance from each other.

A hollow core board 20 whose middle layer 20.3 is constituted solely byjoining elements as described further above may be used as a table top,wherein at locations at which the table legs are to be attached to theunderside of the table top, the hollow core board 20 comprises joiningelements being equipped with means for attaching the legs, e.g. withinner threads into which a treaded bolt can be screwed or with asuitable inner opening into which a corresponding protrusion of thetable leg can be glued. Depending on the stiffness of the outer layers20.1 and 20.2 of the hollow core board 20 forming the table top, on thelocations of the leg attachment and on the overall size of the tabletop, additional inserts may be provided at other locations than the legattachment locations or not.

FIG. 4 shows a last exemplified embodiment of the joining elementaccording to the invention. The same as the joining element according toFIGS. 3A and 3B, the joining element according to FIG. 4 is particularlysuitable for being anchored in the two outer layers 20.2, 20.3 of ahollow core board 20 such that the contact face 1.1 constituting theproximal end of the joining element is flush with the outer surface ofthe first outer layer of the hollow core board 20. The region betweenthe wall anchoring portion 3.2 and the distal end 2 comprises a centraltube 2.1 and radial wings 2.2 extending radially into the recesses ofthe wall anchoring portion 3.2 and reaching axially beyond the distalend of the central tube 2.1 and having a thickness which gets smallertowards the distal end. The sealing portion 3.1 together with the wallanchoring portion 3.2, whose circumferential surfaces are equipped inthe same manner as described above for the joining element according toFIGS. 3A and 3B form together a collar on the central tube 2.1 and coverthe wings 2.2 leaving the proximal end of the central tube 2.2 open.

When the joining element according to FIG. 4 are anchored in a hollowcore board 20 as substantially described in connection with FIGS. 3A and3B, a further object can be fixed to the hollow core board 20 bysuitable means fitting into the central tube 2.1 of the joining element.It is also possible to use the proximal opening of the central tube forintroducing a corresponding protrusion on a distal face of a vibratingtool and therewith enabling guidance of the joining element into theopening by the tool.

The joining element according to FIG. 4 proves to be able to be anchoredin a hollow core board 20 with a pull-out resistance which is inrelation to the amount of material in the joining element superior toother joining element forms.

As mentioned already at the beginning, in addition to the embodiments inwhich the sealing portion 3.1 is part of the joining element as shown inFIGS. 1 to 4, it is also possible to carry out the method according tothe invention with a joining element whose wall anchoring portion formsthe proximal end and with a tool whose distal end is formed for thefunction of the sealing portion. Such embodiments of the methodaccording to the invention are illustrated in FIGS. 6 and 7. For acomparison, FIG. 5 illustrates in the same manner the method in which ajoining element with sealing portion is used.

A tool 30 to be used in the method according to the invention is e.g. asonotrode of an ultrasonic device, and comprises a distal face 31adapted to the contact face of the joining element in a per se knownmanner and suitable for a smooth transfer of the vibration from the tool30 into the joining element and for coupling a force from the tool 30into the joining element.

According to FIG. 5, the joining element comprises in the abovedescribed manner a sealing portion 3.2 and a wall anchoring portion 3.1which meet along a border line 6 in a mouth region 3 of the joiningelement. As the above described sealing function is taken over by thesealing portion 3.2 being part of the joining element, the distal faceof the tool 30 does not need to have the same form and size as theproximal face of the joining element. It may be smaller as shown in FIG.5.

According to FIGS. 6 and 7, the sealing function is taken over by adistal (or sealing) portion 30.1 of the tool 30, which for beingsuitable for the sealing function needs to be formed as discussedfurther above for the sealing portion 3.1 of the joining element.According to FIG. 6, the distal face 31 of the tool 30 is positionedagainst the proximal end of the wall anchoring portion 3.2 of thejoining element. The joining element and the tool 30 are to be forcedinto the opening until the border line 6 between the joining element orits wall anchoring portion 3.2 respectively and the tool 30 has reacheda position beyond the mouth. This means that on removing the tool 30,the proximal face of the joining element is positioned not flush withthe object surface 33 but slightly deeper in the opening (at least about0.5 mm). The same applies to the method as illustrated in FIG. 7 withthe difference that, in analogy to the embodiment shown in FIG. 1E, anarrow border groove 6.1 is provided between the wall anchoring portion3.2 and the distal or sealing portion 30.1 of the tool 30, the grooveportion being part of the joining element and carrying the proximal faceof the joining element. For being suitable for carrying out the sealingfunction, the distal face 31 of the tool 30 needs to be larger than theportion of the groove by the groove depth.

Suitable thermoplastic materials for the joining element are e.g. ABS(acrylonitrile butadiene styrene), PA (polyamide), ASA (acrylonitrilestyrene acrylate) or PS (polystyrene).

What is claimed is:
 1. An assembly, comprising an object, the objectcomprising a first outer layer, a second outer layer, and a middle layerbetween the first and second outer layers, said object having an openingwith a mouth, the opening penetrating the first outer layer and themiddle layer, the assembly further comprising a joining element anchoredin the opening, the joining element comprising: a proximal end and adistal end, the proximal end and the distal end being spaced apart fromone another in a direction of an axis, a contact face in the area of theproximal end, the contact face being suitable for being contacted with avibrating tool for applying, to the joining element, mechanicalvibration and a force parallel to the axis, a wall anchoring portionsituated between the proximal end and the distal end, and a tubularportion situated between the proximal end and the distal end andextending distally from the wall anchoring portion, wherein the joiningelement comprises a thermoplastic material at least at thecircumferential surface of the wall anchoring portion and at the distalend; wherein thermoplastic material from the wall anchoring portionanchors the joining element in the first outer layer, and additionallythermoplastic material from the distal edge anchors the joining elementin the second outer layer.
 2. The assembly according to claim 1, whereinthe middle layer has a density substantially below a density of thefirst outer layer and a density of the second outer layer.
 3. Theassembly according to claim 1, wherein the joining element has a smallercross section region situated between the proximal end and the distalend, the smaller cross section region having a smaller cross sectionthan the wall anchoring portion and extending proximally from the wallanchoring portion.
 4. The assembly according to claim 3, wherein thesmaller cross section region is defined by a groove running around acircumference of the joining element.
 5. The assembly according to claim1 wherein the object is a sandwich board.
 6. The assembly according toclaim 5, wherein the object is a hollow core board.
 7. The assemblyaccording to claim 6, wherein the middle layer is constituted by ahoneycomb structure of cardboard.
 8. The assembly according to claim 5wherein the first and second outer layers are made of wood or chipboard.